The invention relates to an FM-receiver for receiving an FM-signal with transmission identification. An aerial input is connected to a tuning unit to which there are connected, in succession, an IF unit, an FM-detection circuit, a pilot regeneration circuit, a demodulation arrangement for demodulating a code signal which comprises transmission identification information. A clock regeneration circuit is connected to both the pilot regeneration circuit and the demodulation arrangement and comprises a resettable phase search circuit for producing a clock signal the frequency of which is derived from a regenerated pilot frequency and the phase of which is derived from the demodulated code signal to provide clock-controlled decoding circuit for decoding the code signals and a clock-controlled signal processing unit.
Such an FM-receiver is disclosed in Netherlands patent application No. 8000607, which has been laid open to public inspection.
The information for the transmission identification is transmitted in the form of continuously repeated digital code words. The consecutive code words form together a code signal which is binary phase-modulated on what is commonly referred to as a code sub-carrier in the spectrum of an FM-signal. The frequency of this code sub-carrier is in a given, fixed relationship to the frequency of a pilot e.g. the (19 KHz) stereo pilot or the (57 KHz) traffic pilot which is included in the transmitted FM-signal. The frequency of the clock signal with which coding of the code signal has been effected in the transmitter is also in a given relationship to the frequency of this pilot.
In the prior art FM-receiver the first mentioned frequency relationship is employed for stable demodulation of the code signal. The frequency relationship between the clock signal and said pilot is used in association with the demodulated binary code signal for an accurate frequency and phase synchronization of the regenerated clock signals and, after synchronization, for a direct coupling of the regenerated clock signal to the pilot. As a result thereof, once a clock signal has been synchronized it has a high degree of stability and is only disturbed in the event of a drop-out or a considerable amplitude reduction of the relevant pilot.
In practice, in certain circumstances, more specifically with mobile reception, disturbances may occur in the received FM-signal which in the prior art FM-receiver result in annoying disturbances in the reproduction of the transmission identification information. The disturbances in the reproduction may have two different causes. They may be the result of a phase derangement or phase-slip of the regenerated clock signals owing to a drop-out or a considerable amplitude reduction in the received pilot during a certain period of time. Disturbances of such a type can be eliminated by resetting the clock regeneration circuit, so that the phase synchronization of the clock signal is recovered by means of the phase search circuit. For that purpose, the prior art FM-receiver has been equipped with a manually operable reset button.
However, the disturbances in the reproduction may also be the result of disturbances of the code signal itself. Since the amplitude of the code sub-carrier in the received FM-signal is much smaller than the amplitude of the pilot, the last-mentioned disturbances are of a much more frequent occurrence than the disturbances owing to a phase-slip of the clock signal.
Although it is possible to reduce to a certain extent the number of errors in the decoded signal by means of an error correction circuit, it has been found in practice and particularly with mobile reception that disturbances of the code signal occur with such a frequency and such a long duration that even after a possible error correction annoying and repeatedly occurring disturbances in the reproduction of the transmission identification cannot be prevented from occurring.
Consequently, with the prior-art FM-receiver the reproduction of the transmission identification is unstable and sensitive to noise, while the majority of disturbances in the reproduction cannot be eliminated by operating the reset button.